1. Field of the Invention
The present invention relates to a spread spectrum communication system.
2. Background of the Invention
Spread spectrum communication systems have received attention due to their high frequency efficiency as the number of users of the land mobile communication steeply increases. Among various types of spread spectrum communication, the DS-CDMA cellular system is going to be standardized by an international committee on communication.
The DS-CDMA system is classified into two types: strictly synchronous base stations and asynchronous base stations. The global positioning system (GPS) or other systems are used in the synchronous system for synchronization. The acquisition is easy for the synchronous system only by a synchronization of a long code which is common to every base station. Each base station has a delay time of the long code different from the delay times of other base stations. The peripheral cell search for the hand-over is quick because a mobile station receives a delay information from base stations.
In the asynchronous system, different spreading code sequences are allocated to the base stations from one another. The initial cell (sector) search takes a longer time for the mobile station because it is necessary to identify a spreading code sequence. It takes a lot of time when the spreading code is long. However, the search time may be decreased by information on the spreading code from base stations of adjacent cells, and it is advantageous that synchronization by signals from satellites is unnecessary.
A cell search method for quick acquisition in the asynchronous system is proposed in xe2x80x9cTwo-stage Rapid Long Code Acquisition Scheme in DS-CDMA Asynchronous Cellular Systemxe2x80x9d by Kenichi HIGUCHI, Mamoru SAWAHASH1 and Fumiyuki ADACHI, Technical Report of IEICE, CS96-19, RCS96-12(1996-05). Composite codes are generated by corresponding long codes LC0 to LCX for identifying base stations and short codes SC0 to SCY for identifying channels. The short codes are common in all the cells and the code SC0 is allocated to common control channel (perch channel). The mobile station despreads a first short code SC0 by a matched filter that detects the timing of the long code. Then, the long codes are identified by the matched filter or a sliding correlator. The number of cells search is decreased to be (Length and Phase of spreading code) compared to the number of (Number of spread code lengthxc3x97Number of spread code phases) in the asynchronous system above.
Multi-media communication is required for transmitting a plurality of signals of different transmission rates. In the DS-CDMA cellular system, a type of variable spread ratio and a parallel multi-code type are proposed. Fading compensation is indispensable for these systems for high quality service.
FIG. 47 shows a conventional receiver of a DS-CDMA system having a receiver antenna 101 for receiving a spread spectrum signal, a high frequency receiving portion 102 for converting the spread spectrum signal into an intermediate frequency signal, a divider 103 for dividing the intermediate frequency signal into two signals which are input to multipliers 106 and 107. A signal (cos xcfx89t) of the intermediate frequency is generated by an oscillator 104 to be input to the multiplier 106. A signal from the oscillator 104 is shifted by xcfx80/2 by a phase shifter 105 and is input to the multiplier 107. The multipliers 106 and 107 multiply the divided signals by the signal from 104 and 105, respectively. The outputs from the multipliers 106 and 107 are passed through low-pass filters 108 and 109 so that the base band signal of an in-phase component (I-component) and a quadrature component (Q-component) is extracted.
The I- and Q-components are multiplied at a complex matched filter 110 by PN code sequence supplied from a EN generator 111 so as to be despread. In the multi-path environment, the despread components have a plurality of peaks. The despread I- and Q-components are processed successively by a delay detection circuit 112, a signal level detector 114 and a phase correction portion 116.
The delay detection circuit 112 detects a signal of one path, for example the first path, of a plurality of paths and inputs the signal to a frame synchronization detector 113. The received signal has a pilot symbol already-known and four symbols are included in a slot. The frame synchronization detector 113 judges whether the four symbols are identical to a predetermined delay pattern so as to detect the frame synchronization. The detector outputs a frame synchronization signal to the phase correction portion 116. The level detection portion 114 detects the signal level of the I- and Q-components. The multi-path selection portion 115 selects a plurality of paths with higher power from the maximum power and outputs a signal to the phase correction portion.
The phase correction portion 116 has a plurality of phase correction means corresponding to the number of multi-paths and a selector for inputting the despread I- and Q-components according to the signal from the portions 113 and 115. The despread signal of the matched filter 110 is input through the selector at timing of the phase correction means so that the phase correction means performs fading compensation of the despread signal.
It is necessary in frame synchronization that a multiplication of analog signals is implemented by a large circuit. Accordingly, a lot of electrical power is consumed.
FIG. 48 shows a receiver for a semi-direct conversion for a spread spectrum communication. The receiver has an antenna 200 for receiving a signal of a band-pass filter (BPF) 211, a low noise amplifier (LNA) 212 and a frequency converter 213 for outputting a signal of intermediate frequency, fc, by mixing a local oscillation signal of frequency fL. The frequency fL, is for example, a boundary frequency of a frequency band of the received signal. In the final output, a DC offset is reduced but a frequency offset remains. When the received signal is a spread spectrum signal, despreading is performed after the direct conversion. The direct conversion is more useful for other systems such as PHS than spread spectrum communication systems.
The output of the frequency conversion is in the frequency domain at the positive and negative side on the frequency coordinate. A channel of a frequency exists in the negative domain equal to the frequency of the signal to be received. These two signals are separated by a channel filter having a complex coefficient. The output of the converter 213 is quadrature detected and over-sampled, then transformed by a Hirbert transformation portion 215.
The output of I- and Q-components of the portion 215 is input to the channel filter 216 so that signals of adjacent channels are reduced and input to a demodulator 217. The demodulated signal is reproduced to the data transmitted by a data decision portion 218.
The Hirbert transformation needs rather complicated circuit.
The present invention solves the above-discussed conventional problems and has an object to provide a spread spectrum communication system capable of high speed cell search.
The present invention has another object to provide a spread spectrum communication system applicable to multi-media communication.
The present invention has another object to provide a spread spectrum communication system of high reception quality even during multi-path fading.
The present invention has another object to provide a spread spectrum communication system capable of high speed frame synchronization.
The present invention has another object to provide a semi-direct conversion receiver of a simple and small scale circuit.
According to the present invention, a receiver for receiving a traffic channel and a common control channel has a plurality of matched filters at least one of which is selectively available for the traffic or the common control channel. At acquisition, a plurality of matched filters are used for receiving the common control channel. At the hand-over, a plurality of matched filters are used to receive traffic channels of both the current base station and the base stations in the adjacent cells.
According to the present invention, a quantizing circuit is provided for quantizing an output of a matched filter, and a delay detection circuit is provided for performing the delay detection using the output of the quantizing circuit. An output of the delay detection circuit is successively stored in a plurality of delay circuits so as to be compared.
According to the present invention for the semi-direct conversion, an interference reduction circuit has a real input and imaginary input for receiving an output of a frequency conversion circuit and for receiving a xe2x80x9c0xe2x80x9d input, respectively.